In the manufacture of semiconductor devices, coating processes are performed in which films are deposited onto substrates such as silicon wafers by processes that frequently include chemical vapor deposition (CVD). These processes are usually performed in the sealed vacuum chambers of reactors into which reactant gases are introduced and reacted to produce a coating material that adheres to and produces a thin film on the substrate surface. Many of the reactions are carried out by elevating the surface of the wafer to a reaction temperature of the primary film producing reaction, and otherwise maintaining other parts of the reactor at a lower, near ambient temperature, which prevents the occurrence of the reaction and the deposition of the coating material other than on the surface of the substrate to be coated.
In the chemical vapor deposition of materials such as, for example, titanium nitride (TiN), reactions between the reactant gases may occur at temperatures below the reaction temperature of the substrate. Such reactions may produce reaction products that are of a composition other than that of the coating material deposited on the substrate. These materials often form deposits on components of the reactor. When such deposits form, they may interfere with the operation of the reactor or contaminate the substrates being processed. As a result, it is necessary to periodically clean the reactor of the undesired deposits. Such cleaning may involve the periodic introduction of a cleaning gas into the reactor, the cycling of the reactor through various temperature and pressure cycles, or the disassembly of the reactor to scrub the deposits from its internal components.
In titanium nitride CVD processes, for example, in which the primary reaction is that of titanium tetrachloride (TiCl.sub.4) with ammonia (NH.sub.3), the reaction EQU 6TiCl.sub.4 +8NH.sub.3 .fwdarw.6TiN+N.sub.2 +24HCl
occurs at the surface of the substrate with the temperature of the substrate usually maintained in the range of from 300.degree. C. to 650.degree. C. In such a reaction, a gold colored TiN is deposited onto the heated surface of the substrate and the N.sub.2 and HCl reaction by-products, which are gases, are removed through the vacuum exhaust port of the reaction chamber.
In the TiCl.sub.4 +NH.sub.3 process, the walls of the CVD reactors have been traditionally cooled to approximately ambient temperature, or to temperatures that are at least somewhat less than 200.degree. C. to 250.degree. C. Hot walls, or walls at the same temperature as the substrate, would cause rapid buildup of TiN on the reactor walls and the need to continuously clean the reactor, which could require the use of a nitrogen fluoride (NF.sub.3) plasma treatment and purging after the processing of nearly every wafer. At such near ambient temperatures, however, a portion of the reactant gases combine to form adduct salts of the reactants, such as white or yellow salts of TiCl.sub.4.NH.sub.3, on reactor walls and on other such cool surfaces. While the buildup of these salts is slower than that of TiN on surfaces at the primary reaction temperature, they cannot be removed with an NF.sub.3 plasma. Thus, the formation of the adduct salts requires periodic opening of the reactor and the cleaning of the reactor walls with water.
In the course of TiCl.sub.4 +NH.sub.3 processes, substances such as TiN.sub.x Cl.sub.y compounds form blue and dark purple-blue films on certain internal reactor components that are at intermediate temperatures in the 300.degree. C. to 450.degree. C. range. These substances also require periodic cleaning of the reactor, but are removable with NF.sub.3 plasma.
The NF.sub.3 plasma cleaning of the reactor includes the formation of an NF.sub.3 plasma, with the reactor remaining sealed. Such a plasma dissolves the blue and dark blue deposits of TiN.sub.x Cl.sub.y and the gold TiN.
Because such an NF.sub.3 plasma treatment is not effective to remove the light colored, white or yellow, adduct ammonia salts of titanium tetrachloride and periodic opening of the reactor and a manual cleaning of the internal reactor parts is required, serious problems are caused by a shut down of the reactor, interruption of the vacuum by opening of the reactor, cleaning of the reactor's internal surfaces with water, and a resealing and repurging of the reactor. This cleaning process results in substantial down-time and a costly loss of through-put occurs. Where such a CVD reactor is only one of several processing modules of a multistep processing apparatus, such as a silicon wafer processing cluster tool, a substantial amount of productive use of expensive equipment is lost due to the need to clean one of the processing chambers. The opening of the reactor and the use of a water cleaning procedure tends also to increase particulate contamination that is present when the reactor is restarted. Accordingly, a need exists to minimize the need to clean CVD reactors, particularly those for TiN CVD processes, and to avoid the need to open the reactor when such cleaning becomes necessary.